The field of the invention relates to the diagnostic imaging of the human body and more particularly to positron emission tomography.
Positron emission tomography (PET) is an invaluable, well-proven biomedical imaging modality capable of providing quantitative functional information for studying biochemical and physiological processes in vivo. Owing to its unsurpassed sensitivity, specificity, and quantitative accuracy at the molecular level, it can provide excellent diagnosis ability, staging and selection of the most effective treatment for a wide variety of cancers, heart diseases, and brain disorders. As a research tool, it is believed to be a key component in the emerging field of molecular medicine that is revolutionizing modern medicine. Finally, it can also be utilized for developing high-potency drugs at significantly reduced development cost.
To fully exploit the potentials of PET as a biomedical research tool, a notable recent trend in PET technology is the development of dedicated small-animal systems. With their small size, high resolution, and affordable cost, these systems have received great acceptance in major research institutes. It is expected that, as the size and cost of the systems continue to decrease, and their performances continue to improve, these dedicated systems may eventually become indispensable tools in biomedical research laboratories. In addition, because of the reduced system cost and design complexity, they can also serve as the test-bed of new PET technology.
Another trend in,PET is the development of low-cost, high-resolution whole-body scanners for clinical use. Despite high hopes, PET has experienced a discouraging rate of growth in its initial phase of clinical applications. One major factor contributing to this disappointment is their high cost. However, PET advocates have-recently demonstrated that clinical use of PET can substantially reduce medical cost by eliminating unnecessary procedures. Together with the recent FDA approval of FDG for human studies and the coverage of various PET procedures by Medicare and major health providers, PET has begun to steadily gain grounds in clinics. Even so, the use of PET is likely to be excluded from community hospitals unless its cost can be substantially reduced.
One of the major obstacles in the above-mentioned trends in PET is the presence of parallax errors resulting from events mispositioning due to gamma-ray penetration of detectors when incident at oblique angles. For ringbased systems, parallax errors have at least the following two unfavorable effects on PET: (1) they reduce image resolution and hence decrease quantification accuracy of PET and (2) the errors result in progressive degradation in image resolution as the off-center distance increases. This resolution non-uniformity can produce apparent cold spots and structural distortions, thus resulting in false diagnosis.
The practical implications of these effects are far-reaching for new-generation PET system design. For example, to provide the level of resolution (xcx9c1 mm) and sensitivity needed for imaging gene expression and transfer in animal models, long and narrow detector crystals are needed in PET. This crystal geometry is particularly susceptible to parallax errors. The microPET system developed by Cherry et al. at UCLA employs 2xc3x972xc3x9710 mm3 LSO crystals arranged in a 172 mm-diameter ring. At the center of the field of view (FOV), an intrinsic resolution of 1.8 mm is obtained, but it degrades to xcx9c2.4 mm and xcx9c3.8 mm at 3 cm and 5 cm off the center, respectively. These figures translate into xcx9c33% and xcx9c111% resolution degradation at 0.35RD and 0.58RD, respectively, where RD is the detector ring radius. For systems using BGO, crystals of 30 mm in length are often used. As a result, parallax errors in these systems are even more pronounced: a dedicated BGO small-animal system was recently reported to have xcx9c92% resolution degradation at 0.31RD. Thus, PET systems of xcx9c1 mm uniform resolution cannot be realized unless the issue of parallax errors can be resolved.
Owing to the progressive resolution degradation with the off-center distance, parallax errors also place a strong limit on the FOV radius (RF) for a given RD. For example, the resolution figures of the two systems described above are obtained with RF less than 0.65RD. For brain and whole-body PET systems, the compactness, defined as RF/RD, reported in literature are in the range of 0.5-0.6. consequently, a detector may be regarded as in coincidence with only about two-thirds of the total number of detectors in the same ring, resulting in a considerable waste of detector utilization.
Based on this observation, the CTI/Siemens ECAT ART system is able to avoid the need for almost one-third of the detectors that would be normally required (thereby substantially decreasing the product cost) by mounting two opposing detector arcs on a rotating arm. Unfortunately, this approach compromises system sensitivity since detection solid angle of the system is also reduced. Because of the importance of PET, a need exists for a better method for reducing parallax errors.
A method and apparatus are provided for reconstructing images from data obtained from scintillation events occurring within a projection space of a depth-of-interaction positron emission tomography system. The method includes the steps of identifying a segment of each depth-of-interaction detector of respective pairs of depth-of-interaction detectors detecting the scintillation events of the data obtained within the projection space and estimating a set of sinograms from the data based upon a set of depth-independent point spread functions of the identified segments of the respective pairs of depth-of-interaction detectors.